1. Field of the Invention
The present invention relates to an apparatus and a method for machining a workpiece with a form tool to produce a finished product of a gear shape and measuring it.
2. Description of the Related Art
Gears are important components of a machine for transmitting motion by meshing their teeth together one after another. Particularly, since the gears have capabilities of rotary motion of a precise speed ratio and efficiently transmitting large power with an extremely small structure, they are used in very wide number of fields, from the small gears of measuring instruments, watches, etc., to medium size gears of automobile transmission gears to the large gears used as reduction gears of a several ten thousand horse power marine turbine.
Generally, how little the gears cause vibration and noise to occur depends on how accurately they are machined, and improvement in the machining accuracy have been demanded. Particularly recently, improvement in the machining accuracy of gears to exclude vibration and noise sources is required in the automobile field in which the users attach importance to dwelling ability.
Hitherto, processes such as a machining process with a gear cutter, a forging process, and an electric discharge machining process have been used to form gears; for reasons of productivity, an extruding forging process and a sinter forging process are adopted whereby high-volume production of gears can be performed comparatively easily at low costs. For forging, the accuracy of forming dies directly affects gear accuracy, thus it is important to obtain accurate forging. The forging dies are machined by electric discharge machining which is applied independently of the mechanical strength of forging die material and is capable of accurate machining. An attempt has been made to improve the machining accuracy by various methods. For example, disclosed in Japanese Utility Model Laid-Open No. Sho 61-5528 is an electric discharge machining electrode with a roughing electrode and a finishing electrode located on one common electrode shaft whereby the finished machining accuracy of a gear shape can be improved. Described in Japanese Patent Laid-Open No. Hei 4-152025 is an electric discharge machining process of forging dies that can follow various specification changes of gears by forming the electric discharge machining electrode like a spur gear and changing the thickness of an electrode section formed on the gear tip.
In the conventional electric discharge machining of a gear shape, a workpiece is electric-discharge-machined to form the gear shape while a form electrode for electric discharge machining having the same shape as the gear shape to be formed is moved in the axial direction of the workpiece, namely, the gear shape to be formed. At this time, if the electric discharge machining electrode has a male gear shape, a female gear is formed; if the electrode has a female gear shape, a male gear is formed.
However, in the conventional electric discharge machining process of a gear shape, the form electrode for electric discharge machining moves in the axial direction of the gear for machining while the electrode and the workpiece approach each other at spacing of 0.02-0.05 mm via a processing liquid (insulating liquid such as kerosene or pure water), thus it is diffficult to exclude sludge; the sludge adheres to the work face or the electric discharge machining electrode and lowers insulation of the processing liquid, lowering machining accuracy by an external source. Just after electric discharge machining starts or just before it terminates, namely, when only a part of the electric discharge machining electrode approaches the workpiece, discharge concentrates on a part of the approach region of the workpiece and the electric discharge machining electrode; the discharge condition just after the electric discharge machining starts or Just before it terminates differs from that at the time of stationary machining time, namely, when the entire electric discharge machining electrode and the workpiece approach each ether uniformly. As a result, discharge concentrates around the entrance and exit of the electric discharge machining electrode because the area of the electric discharge machining electrode approaching the workpiece is small, and the workpiece is melted too much, causing the work face to change, resulting in lowering the accuracy of the gear shape.
Further, postprocess steps such as grinding, lapping, and shape measurement are generally required; the workpiece is again chucked at each step and a handling error occurs in addition to a mechanical work error, further lowering the accuracy of the gear shape.